We told people muscle-building switches on. It doesn't.

Itera has a card that shows how the leucine from your meals rises and falls through the day, with a dashed line marking the trigger. Under the chart we wrote a sentence explaining it: once a meal's leucine crosses this line, muscle-building switches on.

A user pushed back. He said muscle-building is not off and then on. It is always happening. The leucine is doing something else.

He was right. So we did the reading, wrote it up with citations, and fixed the app. Here is the honest version.

Muscle protein synthesis never turns off

Your muscle is being built and broken down at the same time, every hour, fed or fasted. In the resting, fasted state synthesis runs at a low baseline and breakdown runs a little higher, so on net you lose a tiny bit of muscle between meals. But synthesis is still running the whole time. There is no off state to switch from (Atherton & Smith, J Physiol 2012).

When you eat protein, two things happen for a few hours: synthesis rises above its baseline, and breakdown is suppressed. Net balance flips positive. You build during that window, then drift back toward baseline. Over a normal day the fasted dips and the fed bumps roughly cancel. Net muscle gain over weeks comes from training plus enough protein tilting that balance positive often enough.

So the accurate one-liner is simple: your body rebuilds muscle around the clock, and a protein meal raises that rebuilding for a few hours.

What leucine actually does

Of all the amino acids in a meal, leucine is the main signal. It is sensed inside the muscle cell by a protein called Sestrin2, and when leucine binds it, it activates a pathway called mTORC1 that drives the post-meal increase in synthesis (Wolfson et al., Science 2016). The other essential amino acids are still required as the actual building material. Leucine is the go signal that sets how big the post-meal response is. It is a trigger for the size of the rise, not a switch for whether building happens.

The threshold is a saturation point, not a switch

You will read that there is a leucine threshold, around 2 to 3 grams per meal, that you need to cross. It is a useful rule of thumb, but the real picture is a curve that rises and then flattens. Around 20 grams of quality protein maximized synthesis after leg training in classic work, with extra protein burned for energy rather than building more (Moore et al., AJCN 2009). After whole-body training, 40 grams beat 20 (Macnaughton et al., Physiol Rep 2016), so the right number depends on how much muscle you worked.

And the leucine-trigger idea is genuinely contested in humans. One systematic review of 31 study arms found 16 supporting it and 15 against, with the support concentrated in older adults and in isolated protein powders rather than young adults eating whole food (Zaromskyte et al., Front Nutr 2021). A clean universal threshold is not settled science. It is an approximation that holds better in some people than others.

The post-meal rise is also self-limiting. Synthesis peaks within an hour or two and settles back toward baseline within two to three hours even if amino acids are still high in the blood, the muscle-full effect (Atherton et al., AJCN 2010). That, more than any clean reset of a threshold, is why several protein-containing meals across the day tend to beat one big dose for the daily picture (Areta et al., J Physiol 2013).

The caveats we kept in

We did not want to trade one oversimplification for another, so the writeup is explicit about the limits:

• Acute synthesis measurements do not equal long-term growth. In one study the acute response to a first workout did not predict 16 weeks of growth in the same people (Mitchell et al., PLoS One 2014).
• Leucine alone is not enough. The full set of essential amino acids is required as material (Wolfe, JISSN 2017).
• Total daily protein, around 1.6 grams per kilogram, is a bigger lever than the timing details (Morton et al., BJSM 2018).
• Our chart is an estimate from your logged meals, not a blood reading.

Why this is the part we care about

It is easy to ship a confident sentence that is wrong. It is harder, and the whole point of Itera, to be the instrument that tells you the true thing, including when the honest answer is that the science is mixed. A user caught us oversimplifying, we checked the literature, and the app is more accurate because of it.

That is the posture: no overclaiming, no theatre, just the real mechanism, explained at the level you need it. The card now says your body rebuilds muscle around the clock, and clearing the leucine line gives that rebuilding its strongest few-hour boost. Which is what actually happens.